High intensity arc discharge vessels employed as light sources are a viable option for producing efficient illumination and they have been used as such for many years. Basically, they come in two forms, usually designated low-pressure or high-pressure depending upon their construction. As a general rule, low-pressure light sources use argon as a part of the arc-generating and sustaining medium. Because these light sources use electrodes defining an arc gap between them they must employ a ballast of one kind or another to control the power flow during two extreme conditions; that is, before starting, the device presents a condition similar to an open circuit and after starting a condition tantamount to a short-circuit.
Because of the unique starting requirements, it has often been the case that a starting aid of one kind or another has been used to help in initiating the start of the arc. With respect to initiating a DBD the starting aids are comprised of two basic types: those that create the DBD in the arc chamber and those that create the DBD outside of the arc chamber. An example of the former is shown in U.S. Pat. No. 3,715,622 and an example of the latter is shown in U.S. Pat. No. 6,222,320. Additionally, a specific form of the latter uses what is termed a “bubble in the press.” Such aids are shown, for example, in U.S. Pat. Nos. 5,323,091 and 5,959,404, both of which are assigned to the assignee of the instant invention. These latter two techniques do create a DBD within the bubble; however, its photons have to travel through the quartz vessel to reach the main electrodes inside the discharge vessel and the quartz absorbs most of the UV radiation, which decreases considerably the effectiveness of the starting aid. Additionally, the DBD in the bubble occurs behind the cathode so there is a visibility problem which limits the effectiveness of this particular starting aid. Also, as a part of the starting aid, a molybdenum foil with sharp edges is used. Because of the sharp edges which promote the electron emission, the foil operates better as a cathode than as an anode. Also, because of the small volume of the bubble, there are few free electrons on the wall of the bubble that would allow the foil to operate as an anode. On the other hand, in the bubble using the foil as a cathode can lead to its deterioration as the foil is exposed to strong ion bombardment during the ignition phase.
Yet another technique is shown in U.S. Pat. No. 6,201,348 wherein the starting aid comprises a return wire forming a coil at the base of a lamp, and creates a low pressure DBD in the buffer gas in the outer jacket. First, the DBD produced is diffuse and is created far from the cathode. Second, the photons generated by the DBD have to travel through the wall of the discharge vessel (which is made of quartz) and can be absorbed, thus decreasing its utility.
While all of these various techniques have proved workable, they still provide suboptimal solution to the problem of reliable ignition and are expensive and/or difficult to fabricate.